A revision that the user cannot access (redaction or suppression) has a non-working diff link in page history/contribs. The problem is there's no way they can paste the link to it for oversighter or arbcom attention either, if it's needed.
A user should be able to get a link for a revision (right click/copy link) to pass on to other admins or oversighters, or if needed as evidence in disputes, even if they cannot see the contents of that revision.
A user who was given such a link by a friend would still not be able to view the revdeleted material unless their account had suitable rights. So making the diff link available (cur | prev) is not a security issue.
That the (cur | prev) links provide valid links even if the user does not have access to see the underlying material.